A common and serious problem in the performance of a printed wiring board (PWB) is crosstalk or unwanted noise passing between nearby conductors belonging to parts of the circuitry that should be electrically isolated from each other. Beyond a tolerable level, circuit noise diminishes the quality and reliability or destroys the functionality of the PWB and the equipment in which it's installed. Two major causes of circuit noise are impedance mismatching and the too-close parallel coupling of conductors. Denser signal conductor routing, use of thinner dielectric between layers, and higher signal propagation speeds all compound the problem, and PWB design technology is increasingly employing all of these.
One design technique for controlling PWB circuit noise is the shielding of signal conductors. Signal layers are alternated with shielding ground or power plane layers, and/or signal conductor paths in different nets on the same layer are separated by shielding conductor paths connected to a power or ground. Another common technique is to spread apart all conductor paths in different nets by a fixed distance determined empirically, by rule-of-thumb, or by some kind of circuit simulation. Both of these conservative design techniques can consume unnecessarily large amounts of board signal layer area and can force the addition of extra signal layers to contain necessary conductor routing. This, of course, increases manufacturing cost, while not really guaranteeing electrical viability. To assure electrical performance, prototype boards must be built and tested. Failed designs must be redesigned, and the process repeated until acceptable electrical performance is achieved, and manufacture of production boards can begin.
A far more effective approach is to predict the electrical viability of the circuitry on a PWB during the design of the board, rather than after building and constructing prototype boards. This allows the designer to optimize both electrical performance and the use of board layer area, without the need for expensive, overly conservative, broad-based design techniques. Some attempts to accomplish this involve the use of CAD tools such as electrical simulation and conductor parallelism auditing, but these techniques fall short of their objective because they either do not take into consideration all possible electrical and physical factors, don't consider the complete PWB in its entirety, or fail to focus adequately on just the electrical noise question. What is needed is a mechanism to control internal PWB noise by employing crosstalk and characteristic impedance auditing on the entire PWB physical design, based on all net electrical characteristics as well as physical conductor routing configurations, in conjunction with careful circuit routing and editing.